.\"
.\" GLE Tubing & Extrusions Library Documentation 
.\"
.TH gleTextureMode 3GLE "3.6" "GLE" "GLE"
.SH NAME
gleTextureMode - set the type of GLE automatic texture coordinate generation.
.SH SYNTAX
.nf
.LP
void gleTextureMode (int mode);
.fi
.SH ARGUMENTS
.IP \fImode\fP 1i
bitwise OR of GLE texture mode flags
.SH DESCRIPTION

In addition to the default glTexGen modes that are supplied by OpenGL,
the tubing library also contains some of its own automatic texture
coordinate generation routines. In addition, user-defined texture coord
generation routines can be supplied.

To use texture mapping with the extrusion library, one must remember to "do the obvious": 
.IP
Enable texture mapping through OpenGL 
.IP
Define and load (glTexImage2D/glBindTexture) a texture 
.IP
If using the routine below, then disable glTexgGen 
.LP
gleTextureMode can be used to set the type of automatic texture
coordinate generation to be used. The argument should be a bitwise-OR
of any of the following flags:
.IP \fBGLE_TEXTURE_ENABLE\fP
If this bit is set, then texturing is enabled. If this bit is NOT set,
then automatic texture coordinate generation is disabled.
.LP
The way in which the automatic texture coordinate generation occurs is
determined by one of the following flags. One and only one of these
should be selected at a time. These tokens are enumerants, not
bit-flags.
.IP \fBGLE_TEXTURE_VERTEX_FLAT\fP
Uses the vertexes "x" coordinate as the texture "u" coordinate, and the
accumulated segment length as the "v" coordinate.
.IP \fBGLE_TEXTURE_NORMAL_FLAT\fP
Uses the normal vector's "x" coordinate as the texture "u" coordinate,
and the accumulated segment length as the "v" coordinate.
.IP \fBGLE_TEXTURE_VERTEX_CYL\fP
Uses u = phi/(2*pi) = arctan (vy/vx)/(2*pi) as the texture "u"
coordinate, and the accumulated segment length as the "v" coordinate.
In the above equation, "vx" and "vy" stand for the vertex's x and y
coordinates.
.IP \fBGLE_TEXTURE_NORMAL_CYL\fP
Uses u = phi/(2*pi) = arctan (ny/nx)/(2*pi) as the texture "u"
coordinate, and the accumulated segment length as the "v" coordinate.
In the above equation, "nx" and "ny" stand for the normal's x and y
coordinates.
.IP \fBGLE_TEXTURE_VERTEX_SPH\fP
Uses u = phi/(2*pi) = arctan (vy/vx)/(2*pi) as the texture "u"
coordinate, and v = theta/pi = (1.0 - arccos(vz))/pi as the texture "v"
coordinate. In the above equation, "vx","vy" and "vz" stand for the
vertex's x, y and z coordinates.
.IP \fBGLE_TEXTURE_NORMAL_SPH\fP
Uses u = phi/(2*pi) = arctan (ny/nx)/(2*pi) as the texture "u"
coordinate, and v = theta/pi = (1.0 - arccos(nz))/pi as the texture "v"
coordinate. In the above equation, "nx","ny" and "nz" stand for the
normal's x, y and z coordinates.
.IP \fBGLE_TEXTURE_VERTEX_MODEL_FLAT\fP
.IP \fBGLE_TEXTURE_NORMAL_MODEL_FLAT\fP
.IP \fBGLE_TEXTURE_VERTEX_MODEL_CYL\fP
.IP \fBGLE_TEXTURE_NORMAL_MODEL_CYL\fP
.IP \fBGLE_TEXTURE_VERTEX_MODEL_SPH\fP
.IP \fBGLE_TEXTURE_NORMAL_MODEL_SPH\fP
These define texture mapping modes that are very similar to those
described above, except that the untransformed vertices and/or normals
are used. As a result, textures tends to stick to the extrusion
according to the extrusions local surface coordinates rather than
according to real-space coordinates. This will in general provide the
correct style of texture mapping when affine transforms are being
applied to the contour, since the coordinates used are those prior to
the affine transform.
.SH OPERATION
To best understand how to use the above functions, it is best to
understand how the tubing is actually drawn. Let us start by defining
some terms. The tubing library "extrudes" a "contour" along a "path".
The contour is a 2D polyline. The path is a 3D polyline. We use the
word "segment" to refer to a straight-line segment of the path
polyline. We also interchangeably use the word "segment" to stand for
the section of the extrusion that lies along a path segment.

The tubing library draws segments one at a time. It uses glPushmatrix()
and glPopmatrix() to orient each segment along the negative z-axis. The
segment starts at z=0 and ends at some negative z-value (equal to the
length of the segment). The segment is then drawn by calling
glVertex3f() (and glNormal3F()) by drawing the 2D contour at z=0 and
again at z=-len. (Of course, if the join style is one of the fancy
ones, then the end-points are trimmed in a variety of ways, and do not
land exactly on z=0, or z=-len, but they do come close). Note that
glBegin() and glEnd() are called around each segment. (Note also that
additional glBegins/Ends may be called to draw end-caps or filleting
triangles for the more complex join styles.)

The obvious way to automatically generate textures is to warp the
glVertex() and glNormal() functions, and compute texture coordinates
based on the 3-space vertex and normal coordinates. This is essentially
what the tubing code does, except that it passes some extra parameters.
The glBegin calls are wrapped, and the integer segment number and the
floating-point length of the segment are passed in. By knowing the
segment number, and the segment length, the texture coordinates can be
adjusted. Knowing the length allows the length to be accumulated, so
that a texture is applied lengthwise along the extrusion. It is this
accumulated length that is used in the FLAT and CYL mapping modes.

For each vertex, not only are the vertex x,y,z coordinates available,
but so is a contour vertex counter indicating which contour vertex this
corresponds to. There is also a flag indicating whether the vertex
corresponds to a front or back vertex (i.e. a z=0 or z=-len vertex).
Again, this info can be used to avoid confusion when drawing the more
complex join styles.
.SH HINTS
Here are a few hints, tips, and techniques:
.IP o
Hint: Confused? RUN THE DEMOS! The best way to understand what all the
different texture modes are doing is to see them in action.
.IP o
Hint: The texture matrix can be used to your advantage! That is, you
can use glMatrixMode(GL_TEXTURE) to control how textures are mapped to
the surface. In particular, you may/will want to use it to to rescale
the V coordinate.
.IP o
The origin of the contour will in general change the vertex x's and
y's, thus changing the texture coordinates.
.IP o
The contour "up" vector will NOT influence the texture coordinates. 
.IP o
For the FLAT and CYL modes, the accumulated length really is the
accumulated length of the segments in modeling coordinates. Unless the
extrusion is very small, this length will probably be much larger than
1.0, and so the resulting texture coordinate will wrap. You will
generally want to rescale the "V" coordinate to make the texture map
fit.
.IP o
If the texture is "swimming" around on the surface in an undesired way,
try using the "MODEL" version of the texture generation flag.
.IP o
Typically, you will NOT want to use the "SPH" versions of the texture
generation engine unless you really, really have an extrusion for which
spherical coordinates are appropriate. Most uses of extrusions are best
handled with the "FLAT" and "CYL" generation methods.
.IP o
User-defined texture generation callbacks are not currently
implemented, but these should be very, very easy to hack in as desired.
It should be easy to let your imagination run wild in here. Look at
texgen.c -- what needs to be done should be obvious, I hope.  When in
doubt, experiment.
.SH BUGS
Multiple threads using GLE share a single texture mode.
.SH SEE ALSO
gleExtrusion, gleSetJoinStyle
.SH AUTHOR
Linas Vepstas (linas@fc.net)
